East Anglia ONE
EMF Assessment
Table 8. Distances at which iE fields generated by AC cables attenuate below Earth’s background field induced by tidal flow (based upon 1m burial)
33kV
Distance (m)
0 75kV 0
3-core 132kV
220kV 0.5 – 1 5 – 10
1-core Trefoil 275kV
<Marginally further
1-core
separated 275kV
< Significantly further
iE fields induced around DC cabling will attenuate to background levels at the same distances as B fields (see Table 7) when considering tidal flow, since both depend on the same tidal flow flowing through the B fields; i.e. when the B field drops to the same level as the geomagnetic field, the iE field will also drop to the same level as the background electric field. These distances might be increased when considering fast-moving organisms, should their velocity be greater than the tidal flow considered.
How, or whether, the fields generated by the EAONE project may interact with background fields, and how they may be perceived by electromagnetically sensitive organisms is not certain. The current understanding is that whichever magnetic field (B field or geomagnetic) is more intense is likely to be more easily detected and therefore of greater interest to an organism (Andrew Gill pers. com.). Assuming so, once B fields attenuate to below the geomagnetic field, they may be less relevant to the organisms in question. However, owing to differences in the fields’ geometries and characteristics, and the fact that additive or subtractive interactions may occur, dependent upon their directions, the two fields may still be decipherable.
Similarly, once iE fields generated by AC cables attenuate to below the background (tidally induced) iE field, they may be less relevant to organisms, although owing to differences in geometries and interactions, the two fields may still be decipherable. Contrastingly, background iE fields and those generated by organisms’ movements around DC cables are likely to be of increasingly different magnitude, and therefore more decipherable, with increasing disparity in velocities. Should tidal flow and organism movement be of comparable velocities, the iE fields are likely to be of similar magnitude, and therefore possibly less distinguishable. However, once more, possible additive or subtractive interactions or geometric differences may increase distinguishable characteristics.
4.4. Magnetic anomalies
Another factor which can complicate interpretation of anthropogenic electromagnetic fields is the presence of magnetic anomalies; namely iron-bearing magnetic rocks. If the EAONE project location (Error! Reference source not found.) is compared with a map of seabed sediments in the relevant area (Figure 6), it appears the majority of sediment the cables are likely to be laid in/on will be medium sand, although there are patches of stony substrata. Site specific surveys of the East Anglia zone support the predominance of sandy gravel and gravelly sand with occasional areas of cobbles and boulders (MESL 2011). Whilst areas of medium sand would be unlikely to have significant relevance to interpretation of the effects of the EAONE project’s
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